Magnetic flux leakage inspection method and apparatus including magnetic diodes

ABSTRACT

A magnetic flux leakage method and apparatus for inspecting magnetizable test objects such as pipelines are disclosed. Magnetic diodes are connected to give temperature-independent readings of flux leakage and are spaced apart according to the size of the flaws sought to be detected. Groups of diodes are overlapped to provide complete coverage of the area sought to be inspected. Certain of the largest signals received at any one time from the groups of diodes are transmitted to recording means to reduce the number of signals recorded. Diode signals are substracted from each other to determine relative differences in detected fluxes. For inspecting pipelines, the diodes and magnets are mounted on the apparatus by mounting assemblies which pivot about three axes and the diodes and magnets are urged yieldingly toward the pipeline wall by members formed of open cell foam polyurethane. To aid in locating the flaws along the pipeline, signals from magnetic markers placed along the pipeline are detected and recorded with signals from flaws.

United States Patent 1191 Beaver et a].

[ 1 MAGNETIC FLUX LEAKAGE INSPECTION METHOD AND APPARATUS INCLUDINGMAGNETIC DIODES [75} Inventors: Ruby C. Beaver, Houston, Texx,

Theodor Clasen, Wienhausen; Wolfgang I-Ienning, Celle both of Germany;Emil S. Johnson, Houston, Tex.

[73] Assignee: Vetco Offshore Industries, Inc.,

Ventura. Calif.

[22] Filed: Mar. 15, 1974 [21] Appl. No.: 451,505

Related US. Application Data [63] Continuation-in-part of Ser No. 360l6l May 14,

I973. abandoned,

1 Aug. 12, 1975 Primary ExaminerRobert .1. Corcoran Attorney, Agent, orFirm.loe E. Edwards; George E Glober, Jr.; William L. LaFuze [57]ABSTRACT A magnetic flux leakage method and apparatus for inspectingmagnetizable test objects such as pipelines are disclosed Magneticdiodes are connected to give temperature-independent readings of fluxleakage and are spaced apart according to the size of the flaws soughtto be detected. Groups of diodes are overlapped to provide completecoverage of the area sought to be inspected. Certain of the largestsignals received at any one time from the groups of diodes aretransmitted to recording means to reduce the number of signals recorded.Diode signals are substracted from each other to determine relativedifferences in detected fluxes. For inspecting pipelines, the diodes andmagnets are mounted on the apparatus by mounting assemblies which pivotabout three axes and the diodes and magnets are urged yieldingly towardthe pipeline wall by members formed of open cell foam polyurethane Toaid in locating the flaws along the pipeline signals from magneticmarkers placed along the pipeline are detected and recorded with signalsfrom flaws.

19 Claims, 21 Drawing Figures PATENTED AUG 1 2 I975 SHEET FT JS P\. so.

F a. -2 5 D if! V2.4

GALVONOMETER RECORDER @DBCRHMNATOR DRNER -GALVONOMETER PATENTEB AUG 1 21975 SHEET MAGNETIC FLUX LEAKAGE INSPECTION METHOD AND APPARATUSINCLUDING MAGNETIC DIODES PRIOR APPLICATION This is acontinuation-impart of application Ser. No. 360,161, filed May 14, 1973and now abandoned.

BACKGROUND OF THE INVENTION l. Field This invention relates to detectingflaws in magnetizable test objects by measuring the magnetic fluxleakage caused by such flaws. The invention particularly relates toinspection apparatus which may be referred to as inspection pigs andwhich are adapted to be run through and to inspect buried pipelineswhich transmit petroleum products and other fluids. These inspectionpigs are designed to provide information about the pipelinesdeterioration (such as pits and cracks caused by corrosion, stress orother causes) without the relatively great cost of unearthing thepipeline. This information is particularly important because buriedpipeline is expensive and because a pipeline explosion can destroyadjacent property and take human lives.

2. Prior Art As used throughout this description and in the claims, theterms magnetize" and magnetizing" and related terms refer to the conceptof introducing a magnetic field or magnetic flux into a region orobject. The inspection of a magnetizable test object by magnetizing theobject and measuring the magnetic flux leakage adjacent the object andcaused by flaws in the object is relatively old in the art. For example,U.S. Pat. No. 1,867,685 1932) to Sperry discloses such a device forrailway rails. Further, inspection pigs for inspecting variousconditions in pipelines have been known for over ten years. See, forexample, U.S. Pat. No. 2,782,370 (1957) to Ver Nooy and U.S. Pat. No.3,064,127 (1962) to Green et al.

Inspection apparatus generally referred to as inspection pigs typicallyare propelled through a pipeline by the pressure of the fluids thereinand thus may go through a pipeline without substantially disrupting itsoperation. These pigs frequently comprise one or more supports, annularcups attached to the supports and engaging the inner wall of thepipeline, magnets mounted on a support for magnetizing the pipeline,detectors mounted adjacent the magnets for measuring the fiux leakage,and recorders, such as strip charts and pens, mounted on a support forrecording the flux leakage measurements.

In the known prior art, several basic sensing means or detectors havebeen used to sense flux leakage. Moving a current-conducting wirethrough a magnetic field will induce in the wire electric currentproportional to the speed of movement and the strength of the magneticfield. This type of detector has broad application but has the obviousdisadvantages that the wire must be moving with respect to the magneticfield to produce a reading and that the amplitude of the reading will beproportional to the velocity of the wire. Detectors such as Hallelements and magnetometers do not depend on movement with respect to themagnetic field, but these devices are relatively complex and expensive,require relatively complex and expensive complimentary electroniccircuitry and must be driven by a power source which adds weight andexpense to the inspection device.

Magnetic diodes are relatively new semi-conductor devices which changetheir internal electric resistance as a function of an external magneticfield. They measure magnetic flux independently of their velocity withrespect to the flux and are from 10 to times more sensitive than Hallelements. Magnetic diodes are relatively simple and inexpensive andrequire relatively small amounts of power; however, they are quitesmall, typically having a length of no more than one-quarter inch.Magnetic diodes may be connected in pairs to form twin diodes whichfunction independently of temperature. Magnetic diodes are known to havea variety of applications suitable to their small size, such asgenerating pulses as disclosed in U.S. Pat. No. 3,689,836 (1972) toSnyder, providing a commutating device for a brushless direct currentmotor as disclosed in U.S. Pat. No. 3,688,172 (1972) to Sieber et al.and detecting flux leakage from relatively small areas of movable testobjects which are translated and rotated with respect to the diodes, asdisclosed in U.S. Pat. No. 3,670,239 (1972) to Shiraiwa et al. However,because of their small size, magnetic diodes have no obvious applicationwith test objects such as pipelines that are relatively immovable andrelatively large as compared to the usual one-quarter inch size of saiddiodes.

In the prior art, problems have been encountered in maintaining themagnets and detectors in sufficiently close proximity to the test objectto magnetize the test object and to detect magnetic flux leakagetherefrom caused by defects therein. These problems are particularlyacute with pipelines because the magnets and detectors should bemaintained close to the pipeline wall, preferably at a constant distancetherefrom, but should also be capable of moving radially inwardly andoutwardly responsive to variations in the internal diameter of thepipeline without damaging the pipeline to accommodate decreases in thediameter of the pipeline caused by dents, welds and other obstructions.In the prior art, various spring mechanisms have been used to urge themagnets and detectors toward the pipeline, but these mechanisms havebeen unduly heavy and bulky, are subject to failure, require maintenanceand are relatively expensive.

In the prior art, problems have been encountered in determining thelocation of pipeline flaws detected by a moving flux leakage inspectionapparatus or pig, because the pig seldom travels through the pipeline atan even or known speed due primarily to changes in the terrain andpressure within the pipeline. Odometers attached to the pig inengagement with the interior wall of the pipeline can supply valuableinformation, but have been inaccurate because of slippage. U.S. Pat. No.3,064,127 (1962) to Green et al. discloses the use of radioactivemarkers placed at selected locations along a pipeline and U.S. Pat. No.3,116,457 (1963) to Schmidt discloses the use of coil markers placedalong a pipeline to influence eddy currents, but such markers clearlyare not useful for inspection pigs depending on flux leakage detection.Magnetic markers placed at selected locations along the exterior of thepipeline to influence flux therein have many advantages, but in theprior art their signals were easily confused with signals caused bystopples, nipples and other hardware attached to the line and thusseparate equipment has been required for detecting marker signals,thereby adding to the weight and cost of the pig.

SUMMARY Applicants solve the problems associated with the flux detectorsby providing flux sensing means or detectors which comprise a pluralityof spaced apart twin magnetic diodes. These twin magnetic diodespreferably comprise at least two separate groups. The groups are spacedapart in overlapping relationship so that, while a substantial surfacearea is inspected by one passage of the diodes near the test object,even the smallest flaw sought to be detected will be detected by atleast two twin diodes in one of the groups, and thus will give rise to asignificant signal. Thus, the detectors function independently of theirvelocity with respect to the test object, have great sensitivity, arerelatively simple and inexpensive and require relatively small amountsof power.

Applicants solve the problem of yieldingly maintaining the pig magnetsand detectors in sufficiently close proximity to the interior wall ofthe pipeline to magnetize and inspect the pipeline by providing for themagnets and detectors mounting assemblies which pivot about three axesand by providing two annular members of open cell foam polyurethane,foam rubber or a similar elastomeric material. These members are mountedon the inspection apparatus yieldingly to urge the magnets and detectorstoward the interior wall of the pipeline. The open cell foampolyurethane or similar material is light, inexpensive, easy to maintainand does not require the space of an expanding and contracting springmechanism.

Applicants solve the problem of detecting, without additional sensingmeans or detectors, magnetic markers placed at selected locations alongand exterior to the pipeline by placing the markers with selectedorientations with respect to the pipeline so that the markers will havea predictable influence on the flow of magnetic flux measured by themagnetic diodes in the interior of the pipeline. In the preferredembodiment, the markers are magnets oriented in opposed relationship tothe pig magnets so that as the pig magnets pass by the markers, the fluxmeasured by the magnetic diodes first increases, then decreases and thenincreases again, as the markers alter the flow of flux between the polesof the pig magnets. The magnetic diode signals from the reduction influx flow caused by the markers are processed separately from the diodesignals from flux variations caused by flaws and then are recordedseparately from but adjacent to the flaw signals. Thus, no additionaldetectors are needed to detect the markers.

An object of this invention is to provide a magnetic flux leakageinspection apparatus having sensing means or detectors which aresensitive, simple, inexpensive, which have low power requirements, whichcan measure magnetic flux leakage independently of the speed of theapparatus with respect to the flux leakage, which can inspect relativelylarge surface areas at one time and which can inspect immovable testobjects.

Another object of the invention is to provide an electromagneticpipeline inspection device having magnets and detectors wherein themagnets and detectors are mounted on the device by mounting assemblieswhich pivot about three axes and wherein the magnets and detectors areyieldingly urged against the interior wall of the pipeline by a light,compact, easy-to-maintain and inexpensive member which keeps them inclose and constant proximity to the interior wall of the pipeline butpermits the passage of the device through narrow portions of thepipeline.

Another object of the invention is to provide a method and apparatus fordetecting magnetic markers along the pipeline to provide information asto the location of the detected flaws without the necessity of providingadditional sensing means to detect the markers.

Other objects will be apparent from the drawings, the specifications andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings wherein like referencenumerals indicate like parts and wherein the illustrative embodiments ofthis invention are shown:

FIG. 1 is a view partly in elevation and partly in section of a pipelineinspection apparatus embodying the invention and in place within thepipeline;

FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1;

FIG. 3 is an enlarged, longitudinal, cross-sectional view taken throughthe center of one of the magnet assemblies and its mounting assembly;

FIG. 4 is an enlarged, exploded, perspective view showing part of amounting assembly for a magnet assembly;

FIG. 5 is an enlarged view partly in elevation and partly in sectionshowing one of the magnet assemblies and its mounting assembly;

FIG. 6 is a view in elevation taken along line 6-6 in FIG. 5;

FIG. 7 is a cross-sectional view taken along line 77 in FIG. 3;

FIG. 8 is a schematic illustration of one of the magnetic diodes;

FIG. 9 is a schematic illustration of a configuration of the magneticdiodes and the electronic circuitry associated therewith, showing thediodes connected to the amplifiers according to the techniqueillustrated in FIG. 16;

FIG. 10 is a schematic illustration of another embodiment of theinvention showing the position of the magnetic diodes with respect tothe interior wall of the pipeline and the associated electroniccircuitry which functions to eliminate the effects of unwanted magneticflux background and which connects the diodes to the amplifiersaccording to the technique illustrated in FIG. 16;

FIG. 1 l is a schematic illustration of another embodiment of theinvention comprising a configuration of the magnetic diodes whereby themagnetic fluxes at two longitudinally spaced-apart points along theinterior wall of the pipeline may be measured and compared and wherebythe diodes are connected to the amplifiers according to the techniqueillustrated in FIG. I6;

FIG. 12 is a schematic illustration of the electronic playback circuitryfor visually presenting the signals from the magnetic diodes;

FIG. 13 is a view partly in elevation and partly in section of theplacement of a magnetic marker with respect to the pipeline and withrespect to the inspection apparatus for locating flaws detected by theapparatus;

FIG. 14 is a view in elevation of a strip chart recording of signalsfrom the diodes;

FIG. 15 is a schematic illustration showing a relatively simplearrangement of the magnetic diodes connected to the amplifiers accordingto the technique illustrated in FIG. 16;

FIG. 16 is a schematic illustration showing the details of one techniquefor connecting the magnetic diodes to the amplifiers;

FIG. 17 is a schematic illustration showing the details of the preferredtechnique for connecting the magnetic diodes to the amplifiers;

FIG. 18 is a schematic illustration of the preferred configuration andconnection of the magnetic diodes to the amplifiers;

FIG. 19 is a schematic illustration of another embodiment of theinvention showing the position of the magnetic diodes with respect tothe interior wall of the pipeline and the associated electroniccircuitry which functions to eliminate the effects of unwanted magneticflux background and which connects the diodes to the amplifiersaccording to the technique illustrated in FIG. 17;

FIG. 20 is a schematic illustration of another embodiment of theinvention comprising a configuration of the magnetic diodes whereby themagnetic fluxes at two longitudinally spaced-apart points along theinterior wall of the pipeline may be measured and compared and wherebythe diodes are connected to the amplifiers according to the techniqueillustrated in FIG. 17; and

FIG. 21 is a schematic illustration showing a relatively simplearrangement of the magnetic diodes connected according to the techniqueillustrated in FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an inspectionapparatus or pig embodying the invention and adapted to be run through apipeline PL in the direction indicated by the arrows. The apparatusincludes three sections: a battery capsule A at the downstream end, amagnet module B in the central portion of the apparatus and a recordercapsule C at the upstream end. These three sections are joined togetherin tandem by a pair of universal joints 16 which enable the device tonegotiate relatively sharp bends in the pipeline.

The battery capsule A carries sufficient batteries to provide thenecessary power for the other components of the apparatus or pig. Themagnet module B contains magnets for magnetizing the pipeline andsensing means or detectors for detecting magnetic flux in the pipeline.The recorder capsule C houses equipment for processing and recordingsignals from the detectors. Conductor means 17 which are shown only inpart are provided for conducting electric current and signals throughplugs 18 and between the battery capsule A, the magnet module B and therecorder capsule C.

The battery capsule A includes a support or mandrel 19, a bumper nose 20mounted on the downstream or forward end of the mandrel l9, andspaced-apart plates 21 attached by suitable means, such as welding, tothe ends of mandrel I9. Annular cups 22 are mounted on the plates 21 bysuitable securing means, such as bolts 23. The cups 22 obstruct theannulus between the mandrel l9 and the interior wall of the pipeline PL;these cups support the battery capsule A in the center of the pipelinePL and, together with other elements of the pig, provide resistance tothe flow of fluid through the pipeline and thus constitute propellingmeans for moving the pig through the pipeline when said fluid impingeson the pig.

A plurality of vanes 25 are mounted on the mandrel 19 by spring legs 26which yieldingly extend the vanes toward and into contact with theinterior wall of the pipeline PL. As shown, each spring leg 26 isconnected at each end to lugs 27 on the mandrel 19. The vanes 25 and thespring legs 26 constitute rotator assemblies which gradually rotate thepig about its longitudinal axis as it travels through the pipeline, thuspromoting even wear of the pig.

The magnet module B includes a longitudinally extending central supportor mandrel 28. On each end of said mandrel are mounted plates 29 andcups 30 similar in size and function to the plates 21 and cups 22 of thebattery capsule A. Annular plates 31 and 32 are mounted by suitablemeans, such as welding, at the downstream end and midsectionrespectively of mandrel 28 and form annular bases for pivotally mountingon the mandrel 28 sixteen magnet assemblies 33 in two sets of eightmagnet assemblies each. Each set is located in an annular path about themandrel and can be said to form a ring of magnet assemblies. Each ofthese two sets extends along the entire inner wall of the pipeline, asis seen best in FIG. 2. As shown in FIGS. 1 and 2, the two sets ofmagnet assemblies 33 are circumferentially offset from each other byabout 22 /2 so that the 16 magnet assemblies will cover the entire wallof the pipeline as the apparatus or pig passes through the pipeline.Applicants have found that the turning radius of the pig may be reducedeven further when appropriate by providing a mandrel 28 which comprisestwo sections joined by a suitable universal joint not shown in thedrawings, but mounted between the two sets of magnet assemblies, justdownstream of annular plate 32.

The recorder capsule C similarly includes a central support or mandrel201, cups 202 and plates 203, similar in size and function to those ofcapsule A and module B.

If desired, an odometer may be mounted on the pig for the purpose ofaiding in the determination of the location of pipeline flaws detectedby the pig. As discussed above, the use of such odometers is old and isnot completely satisfactory because of slippage. However, an odometermay be a material aid in locating flaws, particularly when used incombination with other techniques such as magnetic markers. Applicantshave discovered that the use of an odometer of the general charactershown in US. Pat. No. 3,732,625 to Ver Nooy can be appropriate. Thisodometer, not shown in the drawings, comprises an arm having an innerend and an outer end. The inner end is mounted pivotally to the upstreamand of mandrel 201 behind the upstream cup 202. The outer end carries arotatable wheel whose rim engages the interior wall of the pipeline PLupstream of mandrel 201. Springs attached to mandrel 201 and to the armcontinually urge the wheel against the pipeline PL. A small magnet isfixed to the wheel and magnetic flux sensing means such as mag neticdiodes are mounted on the arm so that the magnet will pass by thesensing means once every time the wheel completes a revolution. Thus,the sensing means will generate a signal for every revolution. These signals, which of course are indications of the distance traveled by thepig, are recorded by the recording apparatus described below and may bedisplayed on a strip chart with flaw signals.

Each of the magnetic assemblies 33 is pivotally mounted on the mandrel28 by a mounting assembly which is adapted to pivot about three axes sothat the magnet assembly can follow the interior wall of the pipeline.Each such mounting assembly includes a dual pivot block 34, shown inplace in FIGS. 2, 3 and and shown separately in perspective in FIG. 4.The pivot block 34 has a central bore 35 and has two coaxial pins 36whose axes are substantially perpendicular to the axis of said bore 35.A threaded pivot pin 37 extends through bore 35 and not only secures theblock 34 to the magnet assembly 33, but also enables the magnet assembly33 to pivot about the longitudinal axis of the pin 37. The threads onpivot pin 37 may thus be said to be means for securing pivot block 34 tothe magnet assembly 33. Each of a pair of spaced arms 38 has one of itsend pivotally connected to one of said pivot block pins 36 and theopposite end pivotally connected to one of the spaced lugs 39 by lug pin40. The lugs 39 are secured to the plates 31 and 32 by suitable meanssuch as nut and bolt assemblies 41 and thus the support 28 may be saidto have lugs 39 thereon. Thus, each magnet assembly 33 can pivot aboutthe three longitudinal axes of pins 36, 37 and 40.

Two annular resilient members 42 and 43, which are preferably made ofopen cell foam polyurethane, but may be made of foam rubber or similarelastomeric material, are mounted on the support or mandrel 28 betweensaid mandrel 28 and each set of magnet assemblies 33. These polyurethanemembers 42 and 43 yieldingly urge the magnet assemblies 33 toward saidpipeline PL and into contact with the interior wall of the pipeline PL,but permit the magnet assemblies to swing about pins 36, 37 and 40 andundergo radial movement inwardly and outwardly to accommodate variationsin the internal diameter of the pipeline.

FIGS. 3, 5, 6 and 7 show the detailed structure of one of the magnetassemblies 33. Each assembly includes a junction plate 45 which issecurely attached to magnetizing means such as U-shaped magnet M bybolts 46 and 47. Pivot pin 37 is threaded into a bore in the junctionplate 45 and thus pivotally secures the pivot block 34 to the plate 45.For the purpose of locking the pivot pin 37 to the plate 45, set screw48 impinges on the threads of the pivot pin 37. Bolts 46, together withbolts 49, also secure to the junction plate 45 a resilient bumper 50which thus may be said to be secured to the downstream end of magnetassembly 33 and which protects the magnet assembly 33 against obstaclessuch as icicle welds projecting into the interior of the pipeline. Abottom plate 51 is secured to the junction plate 45 and to a spacer bar52 by bolts 53. Bolts 54 attach the spacer bar 52 to the magnet M. Apair of side plates 55 are fastened to the magnet M by bolts 56. Thus,junction plate 45, bottom plate 5] and side plates 55 may be said toconstitute a housing for magnet M. Applicants have discovered that inmany circumstances it is appropriate to weld bottom plate 51 to junctionplate 45, to spacer bar 52 and to an end plate (not shown) mountedacross the upstream end of magnet M. This welding is preferable to theuse of bolts 53, because vibrations may loosen bolts 53. For the samereason. it may be preferable to weld side plates 55 to bottom plate 51and to said end plate (not shown).

The magnet M has a North pole NO and a South pole SO spaced apart fromsaid pole NO. A base 57 is mounted below and between the poles NO and SOby bolts 58. The base 57 is provided with a pair of upward facing,spaced-apart recesses 59 for receiving resilient means such as springs60. A detector holder 61, having a pair of downward facing, spaced-apartrecesses 62 coaxial with recesses 59, is confined by poles NO and SO andis mounted for movement between the pipeline PL and base 57. Thedetector holder 61 engages springs 60 and is urged thereby towardpipeline PL. This detector holder 61 is formed with a third recess 63which faces upwardly and within which the sensing means or magnetic fluxdetectors are disposed. In the preferred embodiment of the invention,the sensing means or flux detectors are twin magnetic diodes D and aremounted on the upper edge of a circuit board 64 which is connected toits associated electronic circuitry by conductor means 17. The circuitboard 64 is retained in the recess 63 in desired orientation by asuitable dielectric potting compound, such as solid polyurethane, whichfills the recess 63. A replaceable wear shim 65 is provided betweendetector holder 61 and pipeline PL to protect the holder and detectorsfrom injurious contact with the interior wall of the pipeline. The shim65 is removably fixed to the holder 61 by bolts 66. In the preferredembodiment, the twin magnetic diodes D are mounted to the recess 63under the wear shim 65 and are spaced along a semi-circular pathsubstantially perpendicular to the longitudinal axis of the pipeline, sothat when the diodes are so mounted in each of the sixteen magnetassemblies 33, the entire wall of the pipeline will be inspected by thediodes. Applicants have discovered that in many circumstances it ispreferable to insert a thin rubber strip, not shown in the drawings,between the shim 65 and the diodes D to protect the diodes fromvibrations and other stresses which occur when the pig travels throughthe pipeline. The magnets M are preferably mounted uniformly in themagnet assemblies 33 so that their north poles NO are either all at thedownstream end of the pig (as illustrated in the drawings) or all at theupstream end, so that they will magnetize the pipeline PL in a uniformdirection. However, clearly there are many other possible configurationsof the diodes and the magnets which do not depart from the spirit of theinvention.

Thus, the polyurethane member 42 and the springs 60 yieldingly urge thediodes D toward the interior wall of the pipeline to increase theirproximity to the flaws and to maintain them at a constant distance fromthe wall of the pipeline, thereby reducing variations in the signalscaused merely by variations in the distance of the diodes from the wallof the pipeline, but at the same time accommodating any variations inthe internal pipeline diameter. Applicants have found that in manycircumstances it is appropriate to provide restraining means connectingthe magnet assemblies 33 and the mandrel 28 in order to prevent themagnet assemblies 33 from pivoting outwardly into side openings, valvesand other widenings in the pipeline. In the preferred embodiment, theserestraining means are straps connected to the upstream ends of themagnet assemblies and to the mandrel 28. These straps, which are notshown in the drawings, may be of leather or other suitable material. Onestrap is provided for each magnet assembly 33. For the purpose offacilitating the attachment of a strap to each magnet assembly 33, apair of spaced-apart ears, not shown in the drawings, is welded to theupstream end of the inner surface of each bottom plate 51. To facilitatethe attachment of the straps to the mandrel 28, a pair of spaced-apartears, not shown in the drawings, is welded to the mandrel 28 near theupstream end of each magnet assembly 33. Each strap has two ends. Oneend is fastened by means ofa pin extending through the end to the twocars on the magnet assembly 33; the other end is fastened by means of apin extending through that end to the corresponding two ears on themandrel 28. Each strap is of such a length as to permit the desiredcontact between its magnet assembly 33 and the wall of the pipeline P,but to prevent the entry of its magnet assembly into side openings andthe like.

In the preferred embodiment of the invention, the diodes are oriented ina well-known manner so that they detect primarily only that component(the horizontal component) of the flux leakage which is substantiallyparallel to the longitudinal axis of the pipeline. Measuring primarilythe horizontal component increases the longitudinal distance over whichthe measurement may be made, because a significant horizontal componentparallels the longitudinal axis of the pipeline for a large distanceinside the pipeline compared with the vertical component which issignificant only relatively near the points of exit and entry from thepipeline. This measurement of the horizontal component is particularlyuseful in detecting those dangerous flaws with significant dimensionsparallel to the longitudinal axis of the pipeline. In addition,measuring primarily the horizontal component makes the magnitude of themeasurement independent of the angle that the flux leaves the pipeline,as opposed to the vertical component which is dependent on that angle.Further, this orientation simplifies the mechanics of mounting thediodes and enables the flux leakage to be measured at a relatively fardistance from the interior surface of the pipeline wall, because thehorizontal component is greatest at some distance inside the pipeline.In contrast, the vertical component extending perpendicular to the wallof the pipeline is maximum at the points of exit and entry from thepipeline. However, the magnetic diodes in any of the embodimentsdisclosed in this description may be oriented to detect primarily thehorizontal component of the flux, primarily the vertical component, orany combination thereof.

FIG. 8 illustrates the wellknown basic structure of a single magneticdiode. This single diode is a semiconductor device which changes itsinternal electrical resistance as a function of an external magneticfield. The diode comprises a germanium block with four zones: at P zone,an N zone. an intrinsic zone I and a recombination zone Rv Negativecharge lifetime is longer in zone I than in zone R so that, if negativecharges are deflected into zone R by means of a magnetic field whencurrent flows through the diode (in the direction indicated by thearrow), then the resistance of the diode increases. This resistance isalso dependent on temperature and this temperature dependence may besubstantially eliminated by the well-knwn technique of connecting a pairof single magnetic diodes electrically in series and arranging them inmagnetic opposition to form a twin diode which may be said to betemperature independent. This technique is described in TeleftmkensSemiconductor Application Report: Magnetic diodes AHY l0 and theirapplication by H. Moser (Edi- 10 tor: AEG-Telefunken, Fachbereich,Halbleiter/Verbrieb D7l Heilbronn, Postfach 1042 W. Germany).

FIG. 9 is a schematic illustration of a preferred arrangement of i6 twinmagnetic diodes and the electronic circuitry associated with one of the16 magnet assemblies 33. Thirty-two single magnetic diodes are disposedin pairs to form 16 twin diodes. As is shown most clearly in FIG. 16,the two single diodes of each twin diode are fixed in magneticopposition in the sense that one single diode in each twin diode is ofone magnetic orientation and the other single diode is of an oppositemagnetic orientation. Further, the two single di odes of each twin diodeare connected electrically in series so that the difference in thevoltage drops across the two single diodes in each twin diode issubstantially independent of temperature, as described above. Thisdifference in voltage drops is a function of the magnetic flux impingingon the diodes and may be referred to as the output signal of the diodes.FIG. 9 shows four groups of twin diodes, the groups being indicatedgenerally at 70, 80, and 100, respectively and each group comprisingfour twin diodes, said twin diodes being represented by cross-hatchedrectangles numbered 71- 74, 81-84, 91-94 and 101-104, respectively.These twin diodes are arranged along a semi-circular path adjacent tothe interior wall of a section of pipe line PL, said pipeline having twoflaws designated 110 and 111 respectively.

FIG. 16 shows the details of the electrical circuitry illustrated inFIG. 9 by showing in detail the wiring as sociated with each of the fourtwin diodes of group 70. In FIG. 16, the individual twin diodes 71-74are shown to include pairs of single diodes identified respectively as71a and 71b, 72a and 72b, 73a and 73b, and 74a and 7417. As discussedabove, the two single diodes of each twin diode are arranged in magneticopposition and are connected electrically in series, as taught inTelefunkens Semiconductor Application Report and as shown in FIG. 16.Lines 105, 106, 107 and 108 extend from twin diodes 71, 72, 73 and 74,respectively and, indicated collectively as 109 in FIGS. 9 and 16,connect twin diodes 71-74 with amplifier 115, as is discussed below. Theother groups 80, 90 and are wired in a similar manner.

Obviously, the four lines 105, 106, 107 and D8 in FIG. 16 could beconnected to a common point that the twin diodes 71-74 are connected inparalle efore their outputs are transmitted to amplifier ll However, theembodiment shown in FIG. 16 is clt to be preferable and this parallelhookup will not be discussed in detail.

FIG. 17 shows the preferred technique which applicants have discoveredfor connecting the magnetic diodes to the amplifiers. In FIG. 17, as inFIGS. 9 and 16, a plurality of single magnetic diodes are disposed inpairs to form a plurality of twin diodes and are arranged in magneticopposition in the sense that one single diode in each twin diode is ofone magnetic orientation and the other single diode of an oppositemagnetic orientation. These twin magnetic diodes are spaced apart andhave generally the same spatial orientation, so that the single diodescan be said to be two sets of single diodes with all of the singlediodes in the first set being generally of one magnetic orientation andall of the single diodes of the second set being generally of anopposite magnetic orientation. For example, single diodes 71a, 72a, 73aand 74a in FIG. 17 can be said to corn prise a first set of singlediodes. Similarly, single diodes 71b, 72b, 73b and 74b can be said tocomprise a second set of single diodes. The single diodes of the firstset are connected in series. The single diodes of the second set areconnected in series. These two sets are connected to each other inseries and the connection between the two sets is connected to theamplifier 115. As a result, the difference between the voltage dropsacross each set of diodes, which difference is the signal fed into theamplifier 1 15, is substantially independent of temperature, but isdependent on the magnetic flux impinging on the diodes and may bereferred to as the output signal of the diodes.

The hookup illustrated in FIG. 17 is preferable to that illustrated inFIG. 16 because the FIG. 17 hookup requires less power and a lesssophisticated amplifier than that shown in FIG. 16. However, eitherhookup may be used within the scope of this invention. Thus, theelectrical connections illustrated in and discussed in connection withFIGS. 16 and 17 may be referred to as electrical connections which aresuch that the output signals of the diodes are substantially independentof temperature.

FIG. 18 shows twin diodes 71-74, 81-84, 91-94 and 101-104 spaced apartas shown in FIG. 9, but wired according to the technique of FIG. 17. Itwill be understood that amplifiers 1l5118 in FIG. 18 are also shown asamplifiers 115-118 in FIG. 9 and are connected to filter 120 and theother components illustrated in FIG. 9.

In FIGS. 9 and 18, flaw 111 is of the minimum size sought to bedetected. Because flaws below a minimum size do not pose a significantthreat to the pipeline and are not required by the U.S. Department ofTransportation to be corrected, one may detect only those flaws equal toor above that minimum size. The present device contemplates detectingprimarily transverse oriented internal and external flaws such as seams,cracks, pits, hard spots and mechanical damage, although clearly theinvention may be used to detect flaws of any orientation. The apparatusis designed primarily to detect each flaw whose transverse dimension isgreater than three-eighths of l inch.

The twin diodes in each group are spaced along the semi-circular pathadjacent the pipeline wall and adjacent each other so that the distancebetween the centers of any two adjacent twin diodes in each group is nomore than approximately one-half the longest linear dimension along saidpath of flaw 111. As illustrated in FIGS. 9 and 18, this lineardimension is transverse to the longitudinal axis of the pipeline. Formost pipelines in use today, detecting flaws of the size described abovewill involve spacing the twin diodes approximately onequarter inch tothree-eighths inches apart, but obviously this spacing and the size andcharacter of the flaws sought to be detected can be changed Withoutdeparting from the scope of the invention. Further, as illustrated inFIGS. 9 and 18, the four groups 70, 80, 90 and 100 of the twin diodesare spaced along the path in overlapping relationship so that at leastone of the twin diodes in each group is between at least two twin diodesof each adjacent group. For example, the group 70 is adjacent only tothe group 80 and twin diode 74 is spaced between twin diodes 81 and 82.The group 80 is adjacent groups 70 and 90; twin diode 81 is between twindiodes 73 and 74 and twin diode 84 is between twin diodes 91 and 92.This spatial relationship is such that as the four groups are movedalong the pipeline, the magnetic flux from each flaw of a size sought tobe detected and passed over by the magnet assembly illustrated in FIGS.9 and 18 will be detected by at least two twin diodes in one of the fourgroups, thus producing a strong signal for recordation andinterpretation. As indicated above, in the preferred embodiment of theinvention, the diodes are placed in each magnet assembly 33 so that theentire circumference of the pipeline will be inspected.

Even in the absence of flaws or external sources of magnetic flux,typically some flux will flow through the diodes and between the polesNO and SO. By definition, zero flux level is that level of magnetic fluxmeasured by the diodes when the pig is in a flawless section of pipelineremote from sources of flux external to the pig. Also by definition,diode signals corresponding to measured flux above zero level arepositive and diode signals corresponding to measured flux below zerolevel are negative. Thus, flaws will cause flux to leak out of thepipeline PL into the region between poles NO and SO and will give riseto positive diode signals.

FIG. 9 also shows in schematic the electronic circuitry associated witheach group of twin diodes. Standard amplifiers 115, 116, 117 and 118 areassociated with the groups 70, 80, and respectively so that the positiveand negative signals from each group are transmitted to one of theseamplifiers. The positive and negative signals from the four amplifiers-118 are then transmitted to a first filter means such as a standardfilter which passes to the next stage at any given time only a selectednumber, such as one or two or three, of the larger of the positivesignals from the amplifiers. In the preferred embodiment, filter 120passes only the largest of these positive signals from the amplifiers.Such filters are well-known in the art, are sometimes referred to asamplitude selective gates and may be purchased from many sources. Forexample, a filter of the character of Fairchild Semiconductor FSA 141 IMmonolithic diode array, made by Fairchild Camera and InstrumentCorporation of Mountain View, California, may be used. Thus, after thefilter 120, there is only one flaw signal channel for each magnetassembly 33. This filtering out of the smaller signals greatlysimplifies the task of processing, recording and interpreting thesignals from the magnetic diodes, with attendant reduction in theamount, cost and weight of equipment required. Recording only thelargest signal at any one time from the four groups of twin diodes ineach magnetic assembly does not materially affect the usefulness of theinformation gathered by the inspection apparatus, because the conditionof the pipeline at a given point therealong can be evaluated accuratelywithout knowledge of the presence or absence of adjacent but lesserflaws.

The signal from the filter 120 is transmitted to a standard voltagecontrolled oscillator 12] for the purpose of converting amplitudevariations in the signal to frequency variations and thereby reducingthe noise introduced by the recording and playback operation, whichoperation will be explained below. This noise constitutes randomcomponents of the signal not associated with flaws. The signal is thentransmitted from the voltage controlled oscillator 121 to a recordingmeans or recorder 122 such as a standard magnetic tape deck. Recorder122 has a plurality of channels and permanently records and storeswithin the inspection apparatus on 16 separate channels the 16 separateflaw signals from the 16 voltage controlled oscillators associated withthe 16 magnet assemblies.

As an aid in determining the location of detected flaws, magneticmarkers are placed at selected locations along and exterior to thepipeline PL. These markers are preferably magnets, but may be of anymaterial, such as permeable iron, which will influence the flow ofmagnetic flux in the interior of the pipeline when the pig magnets Mpass by the markers. For ex ample, In FIG. 13, marker 124 is a permanentmagnet and is oriented with respect to the pig magnet M to reduce themagnetic flux flow which is induced through the interior of the pipelinePL by the pig magnet M. In FIG. 13, pig magnet M and marker 124 areoriented in opposed relationship in the sence that the pole N of marker124 is disposed upstream of pole SO, whereas pig pole NO is disposeddownstream of pole SO. Thus, as the pig magnet M passes by marker 124,the induced flux measured by the diodes D in the interior of thepipeline first increases, then decreases, then increases again, asmarker 124 alters the flux flow between poles NO and SO. As statedabove, zero flux level is defined as that level of magnetic fluxmeasured by the diodes when the pig is in a flawless section of thepipeline remote from external sources of flux (such as magneticmarkers). Also by definition above, diode signals corresponding to fluxabove zero flux level are positive and those corresponding to flux belowzero flux level are negative. As magnet M approaches marker 124, theinteraction between poles NO and NO will cause the flux measured by thediodes to increase above zero flux level because it will cause more fluxto flow in the region of the diodes between poles NO and SO. When themagnet M passes over marker 124, the magnetic flux flowing between polesNO and SO will tend to be shunted through the marker, thus reducingbelow zero level the flux measured by the diodes D. As pole SO passesover pole SO, the interaction between poles SO and SO will cause anotherincrease above zero flux level in the flux measured by the diodes D.Thus, the magnetic markers will give rise to predominantly negativesignals from the diodes, whereas the flaws will give rise topredominantly positive signals.

As explained above, the positive and negative signals from the diodeswill be transmitted through filter 120 which passes only the largestpositive signals from the four amplifiers. However, the signals from oneof the amplifiers in each magnet assembly 33, for example amplifier 117in FIGS. 9 and 18, are tapped before they get to filter 120 and arepassed through a second filter means such as electric diode 123 whichpasses only the negative signals to a third filter means 125. As isillustrated diagrammatically in FIG. 9, this third filter 125 alsoreceives seven other negative marker signals which have been tapped fromthe seven other amplifiers 117 in each of the seven other magnetassemblies 33 in this magnet ring. The filter 125 passes to the nextstage at any given time only a selected number, such as one or two orthree, of the more negative of these marker signals. In the preferredembodiment, filter 125 is of the character of Fairchild SemiconductorFSA l4lOM monolithic diode array and passes only the most negative ofthese marker signals to a standard inverter amplifier 126 which furtheramplifies them and inverts them so that they will be recorded andvisually pres ented as positive signals. As explained above, the flawsignals will be recorded and visually presented as positive signals;this inversion of the marker signals will thus cause all signals to berecorded and visually presented as positive signals, thus simplifyingtheir processing and interpretation. The inverted marker signals arethen passed to a second standard voltage controlled oscillator 127 andfrom there to the recorder 122 for recordation on a channel next to thechannels for the flaw signals. Thus, in the entire inspection apparatus,there are l6 channels for flaw signals with one filter and one voltagecontrolled oscillator for each channel, two channels for marker signalswith one filter and voltage controlled oscillator for each channel, andone recorder for all 18 channels. However, obviously the number ofmagnet assemblies, channels, recorders and other components may bechanged without departing from the spirit of the invention.

In the preferred embodiment, the location of flaws is determined byplacing the magnetic marker 124 in opposed relationship to pig magnet Mto first increase, then decrease, then increase again the flux measuredby diodes D. However, clearly the magnetic markers may be placed alongthe pipeline with any substantially uniform orientations with respect tothe pipeline (and thus with respect to the pig magnets) to causesubstantially uniform influences on the flux detected by the diodes D inthe interior of the pipeline. For example, marker 124 could be orientedso that pole NO is disposed downstream of pole SO so that, as magnet Mpasses by marker 124, the flux measured by the diodes first decreases,then increases, then decreases.

FIG. 12 illustrates diagrammatically the playback processing of thesignals after the run of the inspection apparatus or pig is completed.The signals are transmitted from recorder 122 to a standard amplifier toa standard discriminator 131 to a standard galvonometer driver 132 to astandard galvonometer 133. The discriminator 131 converts frequencyvariations of the signals to amplitude variations for ease of visualinterpretation. The galvonometer converts the electrical signals intovisual form as shown in the strip chart 134 illustrated in FIG. 14.Preferably the galvonometer includes a light beam impinging on alight-sensitive chart, but the galvonometer may include a mechanicalstylus or any other device for marking the strip chart.

Thus, in the preferred embodiment of the invention there will be 19traces on strip chart 134: one for the flaw signals from each magnetassembly, one for the marker signals from each ring and one for thesignals from the odometer. FIG. 14 illustrates only five of thesetraces. Trace 135 shows a typical negative marker signal 136 which, asexplained above, has been inverted by inverter amplifier 126 to appearas a positive signal for ease of processing and interpretation. TracesI37 and 138 show typical flaw indications 139 and 140; traces I41 and142 show typical background noise signals.

In operation, the magnetic markers are placed at selected locationsalong the pipeline and the inspection apparatus is placed in thepipeline. As explained, the magnetic markers are oriented along thepipeline in 0pposed relationship to the magnets in the inspectionapparatus. Fluid pressure in the pipeline propels the inspectionapparatus therethrough and the magnets and magnetic diodes of theinspection apparatus, yieldingly urged outwardly by the polyurethanemembers, traverse the interior wall or surface of the pipeline insufficient proximity to magnetize successive sections of the pipelineand detect flux leakage caused by flaws in the pipeline. The signalsfrom the magnetic diodes are processed and recorded on magnetic tapewithin the inspection apparatus; positive signals indicating flaws arerecorded on l6 channels; negative signals from the magnetic markers areinverted and recorded on two additional channels; signals from theodometer are recorded on a 19th channel. At the end of the run, themagnetic tape is played back, the signals are converted from frequencyvariations to amplitude variations and are recorded in visual form on astrip chart by a light beam galvonometer. The strip chart may then beinterpreted to analyze the condition of the pipeline and the location ofthe detected flaws.

Thus, it can be seen from the foregoing that an invention has beenprovided which inspects immovable test objects by magnetic diode sensingmeans. These magnetic diodes are sensitive, simple, inexpensive, havelow power requirements and can measure fiux leakage independently of thespeed of the diodes with respect to the flux leakage. Further, thediodes and magnets constantly are kept in sufficiently close proximityto the wall of the pipeline to magnetize and test the pipeline by themounting assemblies which pivot about three axes and by the light,compact, easy-to-maintain and inexpensive polyurethane members. Inaddition, the magnetic markers are detected by the same diodes whichdetect the flux leakage caused by flaws without the necessity ofproviding additional sensing means to detect the markers.

ALTERNATIVE EMBODIMENTS FIGS. and I9 disclose alternative embodiments ofthe invention where six groups of three twin diodes each are spaced inoverlapping relationship similar to that of FIGS. 9 and 18, butnonadjacent groups are connected in pairs to determine and record thedifference between the magnetic fluxes detected by said pairs. Thisresult is accomplished by transmitting the signals from said nonadjacentgroups to differential means such as a differential amplifier which willcompare the two signals and transmit only the difference between the twosignals for recordation and interpretation. This technique is useful forfiltering out the effects both of background magnetic flux not relatedto the flaws and of changes in the diode signals caused by changes inthe distance between the diodes and the pipeline wall.

FIG. 10 illustrates the diodes connected according to the technique ofFIG. 16. FIG. 19 illustrates the diodes connected according to thetechnique of FIG. 17.

In the examples in FIGS. 10 and 19, there are six groups of three twindiodes each, indicated generally at 150, 155, I60, 165, 170 and 175.These groups each include twin diodes numbered 151-3, 156-8, 161-3,166-8, l7l-3 and 176-8 respectively. As shown, the twin diodes in eachgroup are spaced along a path adjacent pipeline PL with flaw 180, thesmallest flaw sought to be detected, so that the distance between thecenters of any two adjacent twin diodes in that group is no more thanapproximately one-half the length of flaw 180. Further, as in theembodiments of FIGS. 9 and 18, at least one of the twin diodes in eachgroup is between at least two of the twin diodes of each adjacent group.The signals from groups 150 and 155 are transmitted to differentialmeans such as a standard differential amplifier 181; the signals fromgroups and are transmitted to a standard differential amplifier 182; thesignals from groups and are transmitted to a standard differentialamplifier 183. At any point in time, these differential amplifierstransmit for further processing, recordation and interpretation (asshown in FIG. 9) only the difference between the signals which theyreceive from their respective pairs of groups. As in FIG. 9, a firstfilter means passes only a selected number, such as one or two, of thelarger of the positive signals from the amplifiers 181, 182 and 183. Inthe preferred embodiment, such filter means passes only the largest ofthese positive signals from the amplifiers. Thus, a differential meansis connected to at least two groups of twin diodes, constituting aplurality of twin diodes, and to a recording means for determining andrecording the difference between signals from said two groups, wherebythe difference between the magnetic flux detected by said groups may bedetermined and recorded.

FIGS. 11 and 20 illustrate yet another embodiment of the invention toeliminate the effects of background flux and changes in the distancebetween the magnetic diodes and pipeline wall. In FIGS. 11 and 20,differential means such as standard differential amplifier 184 isconnected to groups 185 and 186, each group comprising four twin diodes.FIG. 11 illustrates the twin diodes connected according to the techniqueof FIG. 16. FIG. 20 illustrates the diodes connected according to thetechnique of FIG. 17. Instead of lying along a continuous curved path,groups 185 and 186 are spaced along substantially parallel, spaced-apartcurved paths and measure the flux at points spaced along thelongitudinal axis of the pipeline PL. Obviously more than one groupcould be spaced along either of the paths. Of course, the signals aretransmitted from amplifier 184 for further processing, recordation andinterpretation as shown in FIG. 9. As in FIG. 9, a first filter meanspasses only a selected number of the larger of the positive signals fromthe amplifiers. Thus, at least two of the groups along the paths areconnected to said differential means, one of said groups being from oneof said paths and the other of said groups being from the other of saidpaths and said differential means is connected to the recording means,whereby the differences between the magnetic flux along the two pathsmay be determined and recorded.

FIGS. 15 and 21 illustrate a relatively simple embodiment of theinvention comprising a plurality of twin magnetic diodes 190 spacedalong a curved path substantially perpendicular to the longitudinal axisof the pipe to detect flaws such as cracks 191 and 192. In FIG. 15, thetwin diodes are connected as in FIG. 16. In FIG. 21, the diodes areconnected according to the teaching of FIG. 17. The signals from twindiodes 190 are transmitted to a standard amplifier 193 and to otherelectric components as illustrated in FIG. 9 and discussed in connectiontherewith. As in FIG. 9, a first filtcr means passes only a selectednumber of the larger of the positive signals from the amplifiers. Thetwin diodes preferably are spaced apart so that the centers of adjacenttwin diodes are no farther apart than approximately the longest lineardimension along said path of the smallest flaw sought to be detected,illustrated as crack 191 in FIGS. 15 and 21, so that the flaw will notpass between said twin diodes and go undetected, but will be detected byat least one twin diode when said twin diodes move over said flaw. Thetwin diodes may. of course. be spaced closer together to increase thechance of the flaws being detected by two or more twin diodes and thusgenerating a stronger flaw signal. For example in FIGS. 15 and 21, ifcrack 192 is considered to be the smallest flaw sought to be detected.then the twin diodes 190 are spaced along a path so that the centers ofadjacent twin diodes are no farther apart than approxi mately one-halfthe longest linear dimension along said path of the smallest flaw soughtto be detected, whereby said smallest flaw will be detected by at leasttwo twin diodes when said diodes move over said flaw.

The foregoing disclosure and description of the in vention areillustrative and explanatory thereof, and various changes in the sizeshape and materials as well as in the details of the illustratedconstruction may he made within the scope of the appended claims withoutdeparting from the spirit of the invention What is claimed is:

1. An inspection apparatus for inspecting a magnetizable test objecthaving at least one surface. said apparatus comprising a support;

magnetizing means mounted on said support for magnetizing said testobject;

sensing means mounted on the support adjacent to said magnetizing meansand for detecting magnetic flux leakage from the test obect caused byflaws in the test objecL said sensing means including a plurality ofsingle magnetic diodes disposed in pairs in magnetic opposition andconnected electrically to form twin diodes whose output signals aresubstantially independent of temperature; and

means cooperating with said support for moving said support to cause themagnetizing and sensing means to traverse the surface of the test objectin sufficiently close proximity to magnetize said test object and toinspect said test object with said sensing means; wherein the twindiodes are connected electrically to form at least two separate groups,each group having a plurality of twin diodes; and the twin diodes ineach group are spaced along a path so that the distance between thecenters of any two adjacent twin diodes in that group is no more thanapproximately one-half the longest linear dimen sion along said path ofthe smallest flaw sought to be detected; and the groups are spaced alongthe path so that each group is adjacent to at least one other group; and

the groups are spaced along the path in overlapping relationship so thatat least one twin diode in each group is between at least two twindiodes of each adjacent group,

whereby said smallest flaw will be detected by at least two twin diodesin one of the groups when said path of twin diodes is moved over saidflaw.

2. The apparatus of claim I including recording means connected to saidsensing means for making a record of the magnetic Flux leakage detectedby the sensing means.

3. The apparatus of claim 2, wherein said twin diodes in each groupcomprise two sets of single diodes, with all of the single diodes in thefirst set being generally of one magnetic orientation and all of thesingle diodes of the second set being generally of an opposite magneticorientation; and

the single diodes of the first set are connected in series; and

the single diodes of the second set are connected in series; and

the two sets are connected to each other in series.

whereby the difference between the voltage across each set of diodes issubstantially independent of temperature. 4. An inspection apparatus fortraveling through and inspecting a magnetizable pipeline. said apparatuscomprising a support; magnetizing means mounted on said support for magnetizing successive portions of said pipeline;

sensing means mounted on the support adjacent to said magnetizing meansfor detecting magnetic flux within the pipeline, said sensing meansincluding a plurality of twin magnetic diodes. each twin diodecomprising two single magnetic diodes; and

propelling means for moving said apparatus through the pipeline to causethe magnetizing and sensing means to traverse the interior wall of thepipeline in sufficiently close proximity to magnetize said pipeline andto detect magnetic flux leakage from said pipeline caused by defects insaid pipeline,

wherein the twin diodes are spaced along a path so that the centersot'adjacent twin diodes are no farther apart than approximately one-halfthe longest linear dimension along said path of the smallest flaw soughtto be detected,

whereby said smallest flaw will be detected by at least two twin diodeswhen said twin diodes move over said flaw.

5. The apparatus of claim 4 including recording means connected to saidsensing means for making a record of the magnetic flux detected by thesensing means. 6. The apparatus of claim 4 wherein said sensing meansare oriented to detect primarily that component of the magnetic fluxwhich is substantially parallel to the longitudinal axis of thepipeline.

7. An inspection apparatus for traveling through and inspecting amagnctizahlc pipeline, said apparatus comprising a support; magnetizingmeans mounted on said support fr netizing successive portions of saidpipelin sensing means mounted on the support ad cent to said magnetizingmeans for detecting mag ictic llux within the pipeline, said sensingmeans including a plurality of twin magnetic diodes, each twin diodecomprising two single magnetic diodes; and

propelling means for moving said apparatus through the pipeline to causethe magnetizing and sensing means to traverse the interior wall of thepipeline in sufficiently close proximity to magnetize said pipeline andto detect magnetic flux leakage from said pipeline caused by defects insaid pipeline,

wherein the twin diodes are connected electrically to form at least twoseparate groups. each group having a plurality of twin diodes; and

the twin diodes in each group are spaced along a path so that thedistance between the centers of any two adjacent twin diodes in thatgroup is no more than approximately one-half the longest linear dimension along said path of the smallest flaw sought to be detected; and

mag

the groups are spaced along the path so that each group is adjacent toat least one other group; and

the groups are spaced along the path in overlapping relationship so thatat least one of the twin diodes in each group is between at least twotwin diodes of each adjacent group,

whereby said smallest flaw will be detected by at least two twin diodesin one of the groups when said path of twin diodes is moved over saidflaw.

8. The apparatus of claim 7 including recording means, connected to saidsensing means for making a record of the magnetic flux detected by thesensing means.

9. The apparatus of claim 8, wherein said twin diodes in each groupcomprise two sets of single diodes, with all of the single diodes in thefirst set being generally one magnetic orientation and all of the singlediodes of the second set being generally of an opposite magneticorientation; and

the single diodes of the first set are connected in series; and

the single diodes of the second set are connected in series; and

the two sets are connected to each other in series,

whereby the difference between the voltage drops across each set ofdiodes substantially independent of temperature. 10. The apparatus ofclaim 8, including differential means connected to at least two of saidgroups of twin diodes and to said recording means for determining andrecording the difi'erence of twin signals from said two groups,

whereby the difference between the magnetic flux is detected by saidgroups may be determined and recorded.

11. The apparatus of claim 8, including filter means connected to atleast one of said groups of twin diodes transmitting only a selectednumber of the larger signals received at any one time from said groupsof twin diodes.

12. An inspection apparatus for traveling through and inspecting amagnetizable pipeline, said apparatus comprising a support;

magnetizing means mounted on said support for magnetizing successiveportions of said pipeline; sensing means mounted on the support adjacentto said magnetizing means for detecting magnetic flux within thepipeline, said sensing means including a plurality of twin magneticdiodes, each twin diode comprising two single magnetic diodes;propelling means for moving said apparatus through the pipeline to causethe magnetizing and sensing means to traverse the interior wall of thepipeline in sufficiently close proximity to magnetize said pipeline andto detect magnetic flux leakage from said pipeline caused by defects insaid pipeline; and

recording means connected to said sensing means for making a record ofthe magnetic flux detected by the sensing means, said recording meanshaving at least two channels; and

a first filter means connected to at least one of said twin diodes andto at least one channel of said recording means to transmit to saidrecording means only a selected number of the larger positive signalsfrom said twin diode; and

a second filter means connected to at least one of said twin diodes andto a different channel of said recording means to transmit to saidrecording means only negative signals from said twin diode, wherein oneof said positive and negative signals is representative of flux leakagecaused by flaws in the pipeline and the other of said signals isrepresentative of flux flow caused by magnetic markers placed near saidpipeline,

whereby both flux leakage from flaws in the pipeline and flux flowcaused by said markers may be detected together by said twin diodes andrecorded on separate channels by said recording means.

13. An inspection apparatus for traveling through and inspecting amagnetizable pipeline conducting fluid, said apparatus comprising abattery capsule including sulficient batteries to supply power for saidapparatus;

a magnet module connected to said battery capsule and including at leastone magnet assembly, said magnet assembly having a magnet to magnetizethe pipeline as said apparatus travels therethrough and having aplurality of twin magnetic diodes to detect magnetic flux therein,

said twin diodes comprising two single magnetic diodes and beingconnected electrically to form at least two separate groups, each grouphaving a plurality of twin diodes; and

the twin diodes in each group being spaced along a path so that thedistance between the centers of any two adjacent twin diodes in thatgroup is no more than approximately one-half the longest lineardimension along said path of the smallest flaw sought to be detected;and

the groups being spaced along the path so that each group is adjacent toat least one other group; and

the groups being spaced along the path in overlapping relationship sothat at least one twin diode in each group is between at least two twindiodes of each adjacent group; and

a recorder capsule connected to said magnet module and including arecorder connected to said twin diodes to make a record of said detectedmagnetic flux; and

a plurality of annular cups secured to said battery capsule, magnetmodule and recorder capsule and engaging said pipeline,

whereby, as said fluid impinges on said apparatus, it

will travel through the pipeline magnetizing successive portions thereofand detecting and recording magnetic flux leakage caused by flawstherein, and

said smallest flaw will be detected by at least two twin diodes in oneof the groups when said path of twin diodes is moved over said flaw.

14. The apparatus of claim 13, wherein the two single diodes of the twindiodes are fixed in magnetic opposition and are connected electricallyin series, whereby the output signals of said diodes are independent oftemperature.

15. The apparatus of claim 13, wherein said twin diodes in each groupcomprise two sets of single diodes, with all of the single diodes in thefirst set being generally of one magnetic orientation and all of thesingle diodes of the second set being generally of the opposite magneticorientation; and

the single diodes of the first set are connected in series; and

the single diodes of the second set are connected in series; and

two sets are connected to each other in series,

whereby the difference between the voltage drops across each set ofdiodes and is substantially inde pendent of temperature.

16. The apparatus of claim 13, wherein the centers of adjacent twindiodes in each group are between onequarter inch and three-eighthsinches apart.

17. The apparatus of claim 13, including a filter in each magnetassembly connected to each of said groups of twin diodes in that magnetassembly and to said recorder to transmit to said recorder only thelargest signal received at any one time from said groups of diodes inthat magnet assembly.

18. The apparatus of claim 13, wherein said twin diodes are oriented todetect primarily that component of the magnetic flux which issubstantially parallel to the longitudinal axis of the pipeline.

19. The apparatus of claim 17, including a second filter in each magnetassembly connected to one of said groups of twin diodes in that magnetassembly and to said recorder to transmit to said recorder only the mostnegative signals received from said group.

1. An inspection apparatus for inspecting a magnetizable test objecthaving at least one surface, said apparatus comprising a support;magnetizing means mounted on said support for magnetizing said testobject; sensing means mounted on the support adjacent to saidmagnetizing means and for detecting magnetic flux leakage from the testobect caused by flaws in the test object, said sensing means including aplurality of single magnetic diodes disposed in pairs in magneticopposition and connected electrically to form twin diodes whose outputsignals are substantially independent of temperature; and meanscooperating with said support for moving said support to cause themagnetizing and sensing means to traverse the surface of the test objectin sufficiently close proximity to magnetize said test object and toinspect said test object with said sensing means; wherein the twindiodes are connected electrically to form at least two separate groups,each group having a plurality of twin diodes; and the twin diodes ineach group are spaced along a path so that the distance between thecenters of any two adjacent twin diodes in that group is no more thanapproximately one-half the longest linear dimension along said path ofthe smallest flaw sought to be detected; and the groups are spaced alongthe path so that each group is adjacent to at least one other group; andthe groups are spaced along the path in overlaPping relationship so thatat least one twin diode in each group is between at least two twindiodes of each adjacent group, whereby said smallest flaw will bedetected by at least two twin diodes in one of the groups when said pathof twin diodes is moved over said flaw.
 2. The apparatus of claim 1including recording means connected to said sensing means for making arecord of the magnetic flux leakage detected by the sensing means. 3.The apparatus of claim 2, wherein said twin diodes in each groupcomprise two sets of single diodes, with all of the single diodes in thefirst set being generally of one magnetic orientation and all of thesingle diodes of the second set being generally of an opposite magneticorientation; and the single diodes of the first set are connected inseries; and the single diodes of the second set are connected in series;and the two sets are connected to each other in series, whereby thedifference between the voltage across each set of diodes issubstantially independent of temperature.
 4. An inspection apparatus fortraveling through and inspecting a magnetizable pipeline, said apparatuscomprising a support; magnetizing means mounted on said support formagnetizing successive portions of said pipeline; sensing means mountedon the support adjacent to said magnetizing means for detecting magneticflux within the pipeline, said sensing means including a plurality oftwin magnetic diodes, each twin diode comprising two single magneticdiodes; and propelling means for moving said apparatus through thepipeline to cause the magnetizing and sensing means to traverse theinterior wall of the pipeline in sufficiently close proximity tomagnetize said pipeline and to detect magnetic flux leakage from saidpipeline caused by defects in said pipeline, wherein the twin diodes arespaced along a path so that the centers of adjacent twin diodes are nofarther apart than approximately one-half the longest linear dimensionalong said path of the smallest flaw sought to be detected, whereby saidsmallest flaw will be detected by at least two twin diodes when saidtwin diodes move over said flaw.
 5. The apparatus of claim 4 includingrecording means connected to said sensing means for making a record ofthe magnetic flux detected by the sensing means.
 6. The apparatus ofclaim 4 wherein said sensing means are oriented to detect primarily thatcomponent of the magnetic flux which is substantially parallel to thelongitudinal axis of the pipeline.
 7. An inspection apparatus fortraveling through and inspecting a magnetizable pipeline, said apparatuscomprising a support; magnetizing means mounted on said support formagnetizing successive portions of said pipeline; sensing means mountedon the support adjacent to said magnetizing means for detecting magneticflux within the pipeline, said sensing means including a plurality oftwin magnetic diodes, each twin diode comprising two single magneticdiodes; and propelling means for moving said apparatus through thepipeline to cause the magnetizing and sensing means to traverse theinterior wall of the pipeline in sufficiently close proximity tomagnetize said pipeline and to detect magnetic flux leakage from saidpipeline caused by defects in said pipeline, wherein the twin diodes areconnected electrically to form at least two separate groups, each grouphaving a plurality of twin diodes; and the twin diodes in each group arespaced along a path so that the distance between the centers of any twoadjacent twin diodes in that group is no more than approximatelyone-half the longest linear dimension along said path of the smallestflaw sought to be detected; and the groups are spaced along the path sothat each group is adjacent to at least one other group; and the groupsare spaced along the path in overlapping relationship so that at leastone of the twin diodes in each group is between at least Two twin diodesof each adjacent group, whereby said smallest flaw will be detected byat least two twin diodes in one of the groups when said path of twindiodes is moved over said flaw.
 8. The apparatus of claim 7 includingrecording means connected to said sensing means for making a record ofthe magnetic flux detected by the sensing means.
 9. The apparatus ofclaim 8, wherein said twin diodes in each group comprise two sets ofsingle diodes, with all of the single diodes in the first set beinggenerally one magnetic orientation and all of the single diodes of thesecond set being generally of an opposite magnetic orientation; and thesingle diodes of the first set are connected in series; and the singlediodes of the second set are connected in series; and the two sets areconnected to each other in series, whereby the difference between thevoltage drops across each set of diodes substantially independent oftemperature.
 10. The apparatus of claim 8, including differential meansconnected to at least two of said groups of twin diodes and to saidrecording means for determining and recording the difference of twinsignals from said two groups, whereby the difference between themagnetic flux is detected by said groups may be determined and recorded.11. The apparatus of claim 8, including filter means connected to atleast one of said groups of twin diodes transmitting only a selectednumber of the larger signals received at any one time from said groupsof twin diodes.
 12. An inspection apparatus for traveling through andinspecting a magnetizable pipeline, said apparatus comprising a support;magnetizing means mounted on said support for magnetizing successiveportions of said pipeline; sensing means mounted on the support adjacentto said magnetizing means for detecting magnetic flux within thepipeline, said sensing means including a plurality of twin magneticdiodes, each twin diode comprising two single magnetic diodes;propelling means for moving said apparatus through the pipeline to causethe magnetizing and sensing means to traverse the interior wall of thepipeline in sufficiently close proximity to magnetize said pipeline andto detect magnetic flux leakage from said pipeline caused by defects insaid pipeline; and recording means connected to said sensing means formaking a record of the magnetic flux detected by the sensing means, saidrecording means having at least two channels; and a first filter meansconnected to at least one of said twin diodes and to at least onechannel of said recording means to transmit to said recording means onlya selected number of the larger positive signals from said twin diode;and a second filter means connected to at least one of said twin diodesand to a different channel of said recording means to transmit to saidrecording means only negative signals from said twin diode, wherein oneof said positive and negative signals is representative of flux leakagecaused by flaws in the pipeline and the other of said signals isrepresentative of flux flow caused by magnetic markers placed near saidpipeline, whereby both flux leakage from flaws in the pipeline and fluxflow caused by said markers may be detected together by said twin diodesand recorded on separate channels by said recording means.
 13. Aninspection apparatus for traveling through and inspecting a magnetizablepipeline conducting fluid, said apparatus comprising a battery capsuleincluding sufficient batteries to supply power for said apparatus; amagnet module connected to said battery capsule and including at leastone magnet assembly, said magnet assembly having a magnet to magnetizethe pipeline as said apparatus travels therethrough and having aplurality of twin magnetic diodes to detect magnetic flux therein, saidtwin diodes comprising two single magnetic diodes and being connectedelectrically to form at least two separate grouPs, each group having aplurality of twin diodes; and the twin diodes in each group being spacedalong a path so that the distance between the centers of any twoadjacent twin diodes in that group is no more than approximatelyone-half the longest linear dimension along said path of the smallestflaw sought to be detected; and the groups being spaced along the pathso that each group is adjacent to at least one other group; and thegroups being spaced along the path in overlapping relationship so thatat least one twin diode in each group is between at least two twindiodes of each adjacent group; and a recorder capsule connected to saidmagnet module and including a recorder connected to said twin diodes tomake a record of said detected magnetic flux; and a plurality of annularcups secured to said battery capsule, magnet module and recorder capsuleand engaging said pipeline, whereby, as said fluid impinges on saidapparatus, it will travel through the pipeline magnetizing successiveportions thereof and detecting and recording magnetic flux leakagecaused by flaws therein, and said smallest flaw will be detected by atleast two twin diodes in one of the groups when said path of twin diodesis moved over said flaw.
 14. The apparatus of claim 13, wherein the twosingle diodes of the twin diodes are fixed in magnetic opposition andare connected electrically in series, whereby the output signals of saiddiodes are independent of temperature.
 15. The apparatus of claim 13,wherein said twin diodes in each group comprise two sets of singlediodes, with all of the single diodes in the first set being generallyof one magnetic orientation and all of the single diodes of the secondset being generally of the opposite magnetic orientation; and the singlediodes of the first set are connected in series; and the single diodesof the second set are connected in series; and two sets are connected toeach other in series, whereby the difference between the voltage dropsacross each set of diodes and is substantially independent oftemperature.
 16. The apparatus of claim 13, wherein the centers ofadjacent twin diodes in each group are between one-quarter inch andthree-eighths inches apart.
 17. The apparatus of claim 13, including afilter in each magnet assembly connected to each of said groups of twindiodes in that magnet assembly and to said recorder to transmit to saidrecorder only the largest signal received at any one time from saidgroups of diodes in that magnet assembly.
 18. The apparatus of claim 13,wherein said twin diodes are oriented to detect primarily that componentof the magnetic flux which is substantially parallel to the longitudinalaxis of the pipeline.
 19. The apparatus of claim 17, including a secondfilter in each magnet assembly connected to one of said groups of twindiodes in that magnet assembly and to said recorder to transmit to saidrecorder only the most negative signals received from said group.